Why bacterial tree diseases matter and what you can do to help
Woodlands cover around 13% of the total land area of the UK. Although this figure is much lower than the European average of 37%, the UK’s trees are nonetheless highly valued for timber production, biodiversity, water and air quality improvement. They also have numerous health and aesthetic benefits for society. Trees have an increasing role in combating climate change threats, helping with carbon sequestration, flood mitigation and urban cooling. Such factors are driving a national ambition to enhance the UK’s woodland resource by greatly increasing the level of tree planting over the next few years.
These ambitions, however, also raise the risk of new tree diseases, including bacterial diseases, being introduced from overseas through plant imports and other pathways involving humans. One such high profile bacterial plant pathogen is Xylella fastidiosa which is now present on mainland Europe and causing disease outbreaks on various plant and tree species. Other bacterial pathogens are out there, some of them currently unknown to science. This blog focuses on native and invasive bacterial pathogens that have already altered our woodlands, orchards, parks and gardens. It also considers how science has helped to reduce their impacts, and how you can do your bit to keep the new ones, like Xylella, out.
Bacterial canker of cherry and plum
If you have fruit trees in your garden you may already be aware of bacterial canker of cherry and plum (caused by Pseudomonas syringae pathovars). It produces those shot holes in the leaves and annoying gummosis on dying branches (see photos below). These bacteria have been present in the UK for a long time. We now know a lot about how they infect trees and what environmental factors are important. However, they still cause very damaging outbreaks in commercial fruit orchards and so we need to know more. Current research in the UKRI Bacterial Plant Diseases Programme is helping us to understand why different variants of these bacteria infect particular host species and cultivars, and whether we can predict this to prevent new host jumps. [Find out more about cherry and plum canker]
Fireblight
As if we didn’t have enough bacterial gumming to deal with on plum and cherry, a new bacterial pathogen of pear and apple trees arrived on our shores from North America in the 1950s – the infamous ‘fireblight’ (Erwinia amylovora). It is so called because of the scorched appearance of infected trees (see photos below). This pathogen also spreads on hawthorn and can be seen in hedges, particularly in England. Research has helped us to understand that early flowering apple and pear cultivars are more likely to ‘escape’ the disease as the pathogen infects through the flowers. Bacterial populations build up during the summer and removal of late blossoms can help reduce infections. Studies of the fireblight pathogen have helped us understand the genetics of how it infects the plant so that resistant apple and pear varieties can be developed. [Find out more about fireblight]
Horse chestnut bleeding canker
And more recently, our beloved horse chestnut – not a UK native but still a highly valued shade tree in parks and gardens across the country – has been dealt a blow by an invasive Pseudomonas syringae bacterium. This disease, known as horse chestnut bleeding canker, is estimated to have killed around a third of horse chestnut trees in the UK since the early 2000s when the epidemic swept through. Many infected trees which survived the initial wave of the epidemic manage to live on with unsightly bleeds and cracks on the bark (see photos below). Science has revealed that the bacterium probably came to Europe from India in a single introduction. Infections largely occur in spring and autumn on young shoots and branches, carried by wind-driven rain. [Find out more about horse chestnut bleeding canker]
What you can do to help
Many plant pathogens, including the horse chestnut bleeding canker bacterium, can survive for long periods in soil, so it is very important to clean all soil and plant debris off boots, tools and tyres when working around infected trees. You can go one better and make cleaning your boots and wheels routine when you finish a walk or cycle in the park or countryside. It might not seem like much but these efforts can really help in reducing spread of plant pathogens within the UK. It is doubly important if you are travelling internationally.
There are other things that you can do to help reduce the risk of importing new bacterial (and other) diseases into the UK. Aim to buy your plants from a nursery which has high standards of biosecurity. Check that the plants you buy are 100% visibly healthy (not discoloured, withered or spotted) and look for plants that have been UK grown for their entire lifespan. You can also consider supporting growers who are members of the UK’s new Plant Healthy Certification Scheme which is being rolled out across the country to help halt the introduction and spread of plant pests and pathogens.
by Sarah Green, Forest Pathologist at Forest Research and Bacterial Plant Diseases Programme Lead